Steering systems such as electronic power steering systems, for example, exhibit deviations between the specified torque determined by the driver of the vehicle and the rotor angle actually set by the steering system. The torque, also called manual torque, is introduced into the steering system via a steering handle such as a steering wheel or a joystick and the like. A desired rotor angle (desired RW) is derivable from the manual torque. In an idealized manner, a linear relationship is assumed between the manual torque and the desired rotor angle. In conventional steering systems, the manual torque is transmitted via a torsion element known as a torsion bar (TB). The driver of the vehicle is supported by an electrical or hydraulic motor or actuator such that the manual torque to be introduced may be lower. The actuator transmits a rotor angle (RW) onto an actuator shaft. In the process, however, the steering system does not follow the specified manual torque completely. The deviations are attributed to numerous causes. Generally, the deviations are attributed to static deviations and dynamic deviations, which, being system-related, are sought in static frictions, in a hysteresis behavior of the steering system, in liquid friction losses, in velocity losses and many other causes.
German Published Patent Application No. 199 56 713 describes an electric power steering device, the control unit of which calculates a current control value for the motor of the power assisted steering device. Deviations of the specified current control value from the actual power steering device control are attributed to additional loads, which are caused by feedback effects of the roadway surface on the rack-and-pinion steering gear. The feedback effects are to be absorbed with the aid of an additional element, an elastic body.
U.S. Patent Application Publication No. 2003/006088 describes a compensation table for the kinetic friction being stored in the engine control unit.
A similar approach is described in German Published Patent Application No. 102 21 678, which attributes the friction in the steering system to a hysteresis torque, which is to be taken into account in the desired torque input. The hysteresis characteristic curve is ascertained as a function of the non-compensated desired torque.
German Published Patent Application No. 199 20 975 subdivides the causes for the friction losses in a more differentiated manner. Five different kinds of friction are distinguished and calculated in terms of control engineering. For this purpose an estimated value is assumed for the static friction. The control provided thus estimates the system deviation of the steering, which has a separately energized DC motor as an actuator.
On the whole, the foregoing steering systems represent an attempt to improve the feel of the steering for a driver of the vehicle. Conventional friction compensations partly have the tendency to overcompensate in the case of steering systems that have little friction.